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Intergranular(IG) fracture due to thermal treatment has been reported in a reactor pressure vessel(RPV) steel of Russian light water reactor in last decade. This is attributed to grain boundary segregation of phosphorus (P) or precipitation of carbide, etc.. This is a finding a difference in microstructure before and after IG cracking; this cannot explain the cause of the IG embrittlement. This old paradigm follows only correlation. Recently, a mechanism in which IG embrittlement occurs due to a decrease in entropy of a material has been reported at a temperature where atomic diffusion is possible. It is anticipated that new paradigm can explain the IG embrittlement of RPV based on a causal relationship. Thus, the thermal treatment at 350-420 ℃ was applied to RPV steel of SA508 and IG cracking was confirmed. DSC analysis was applied to confirm whether a decrease in entropy due to a short range ordering reaction occurs in SA508. It was possible to quantify the entropy change(ΔS= Q/T) through DSC measurement. A lattice changes due to thermal treatment were confirmed using XRD analysis in aged specimens. The results showed that lattice contraction by aging causes a reduction of fracture toughness. The internal stress formed inside the material due to entropy reduction can be calculated by multiplying the exothermic energy per unit mass by the density. This relationship is expressed by a equation of stress(σ) = exothermic heat(ΔQ) x density(ρ). (Received May 21, 2021; Accepted June 28, 2021)

keyword : reactor pressure vessel, RPV, SA508, RPV steel, DSC, differential scanning calorimeter, short range ordering, SRO, exothermic reaction, entropy

This study investigated the influence of partial replacement of Si by Al on the microstructure and tensile properties of ultra-high strength steels with martensite-bainite complex microstructure produced by austenitization and subsequent isothermal heat treatment around M_s temperature. When the isothermal heat treatment was done below the M_s temperature, the fraction of martensite increased with the lower isothermal temperature, but the fractions of constituent phases in the final microstructure were not significantly affected by the partial replacement of Si by Al. Nevertheless, the increase in Al content in the complex phase steel accelerated the bainite transformation, which is thought to be associated with the increase of the free energy difference between FCC and BCC. The enhancement of the bainite transformation not only effectively suppressed the martensite formation upon final cooling when the isothermal temperature was above the M_s temperature but also helped refine the final microstructure when subjected to isothermal heat treatment below the M_s temperature. The yield strengths of the investigated complex phase steels were little influenced by the partial replacement of Si with Al, as long as the fractions of the constituent phases were comparable. This possibly originates with the solid solution hardening and the microstructure refinement with Al addition. (Received June 8, 2021; Accepted June 30, 2021)

keyword : complex phase steels, martensite-bainite, isothermal heat treatment, transformation, hardening

PosMAC^® is a hot dipping Zn-Mg-Al coated steel sheet developed by POSCO. PosMAC^®3.0 shows excellent anti-corrosion performance and is suitable for construction and solar energy systems in severe corrosive environments. PosMAC^®1.5 has a superior surface quality and is preferred for automotive and home appliances. The advanced anti-corrosion properties of PosMAC^® comes from a dense corroded layer which forms on coated surfaces, compared with traditional Zn coatings such as GI, GA and EG. PosMAC^® steels show superior corrosion protection compared to GI coatings in cyclic corrosion tests, despite an approximate 30% reduction in coating weight. The PosMAC^® has excellent application properties for the arc welding of automotive chassis. It has a heat resistance that is more robust than the GI coating, and maintains excellent corrosion protection near the welds of the chassis. Zn-Mg-Al coatings, whose chemical compositions are similar to PosMAC^® coatings, have very low surface friction properties compared to the GI coating. The friction coefficient of PosMAC^® is stabilized to 0.09~0.11. In contrast, the GI coating showed higher friction coefficients of 0.2~0.3 in the repeated friction test. PosMAC^® would be appropriate for complex forming parts with less galling, given these low friction resistance properties. It is expected that the industrial demand for PosMAC^® steel will increase in the near future, thanks to its Zn saving and high anti-corrosion performance. (Received October 9, 2020; Accepted May 24, 2021)

keyword : Anti-corrosion, Zn-Mg-Al, alloy coating, PosMAC^{®1, 5, PosMAC®3, 0}

The effect of 3 forging routes (Route A - 1~12 passes by plane forging (PF) and reverse-plane forging (R-PF), Route B - 1~6 passes by PF and R-PF, 7~12 passes by diagonal forging (DF) and reverse-diagonal forging (R-DF), Route C - 1~12 passes by DF and R-DF) on maximum load to produce the workpiece, deformation heterogeneity and hydrostatic pressure distribution in AA1100 was theoretically investigated using finite element analysis (FEA). The maximum load per pass required to complete 1 cycle of the SPD process was different depending on the forging routes. Route A was relatively higher than Route B and C. From the results of effective strain, the deformation heterogeneity was predicted at the center, edge, and corner regions of the AA1100 workpiece produced by Route A and B. However, the distribution of effective strain in Route C was relatively more homogeneous than Route A and B. The average hydrostatic pressure, which is closely related to the suppression of crack formation in the workpiece under multi-axial forging, was predicted to be relatively bigger in Route C than Route A and B. (Received December 1, 2020; Accepted March 2, 2021)

keyword : AA1100, forging, deformation, multi-axial, finite element analysis

The paper provides experimental details of the welding and specific examples of welding aluminum welding battery cans and conductive tabs for battery pack manufacture. In this study, we provide experimental details of a process for joining dissimilar materials used in sealing battery parts. A laser brazing technique was used for the lap joining of aluminum alloy and a deep drawing quality stainless steel, with an Al-Si filler metal. These materials are commonly used in battery applications, as materials for the cap plate, tab plate and can. The relationships among the width of the brazed zone, formation of intermetallic compounds (IMCs), shape of the joint interface, and joint strength were systematically investigated with respect to the laser power and filler wire feeding rate. When a low and medium laser power (1.2-2.0 kW) was applied, the joint strength was very low, and fracture occurred across the band-shaped IMC layer. With a further increase in the applied laser power (2.2-2.8 kW), a new needle-like IMC composed of Al13Fe5 with a monoclinic crystal structure was formed, and it penetrated the brazed zone. In addition, the width of the brazed zone increased due to the partial melting of the aluminum. The joint efficiency under a high laser power condition was 70% compared to that of the base material. Fractures occurred alternately along the needle-shaped IMC and filler metal zone. Since the fracture propagated along the needle-like IMCs inside the brazed zone, the peak load was higher than that of the band-shaped IMCs. (Received April 21 2021; Accepted June 17, 2021)

keyword : Aluminum, Steel, High power laser brazing, Intermetallic compound, Tensile shear test

A cooling channel with an optimized design provides not only high throughput with gravity die casting, but also guarantees product quality. A conformal cooling channel (CC) whose structure follows the shapes or surfaces of the mold cavity has attracted great attention in the die casting industry, because it allows rapid and uniform cooling. However, implementing conformal cooling remains highly challenging, because the complicated geometries of CC are difficult to form using conventional fabrication methods such as drilling and milling. In recent years, advances in additive manufacturing (AM) technology have made it possible to fabricate products with complex and elaborate structures. In this paper, a gravity die casting mold with CC was designed and built using AM technology. The cooling channel performance was estimated and evaluated using an Al-Si-Cu alloy casting simulation and die casting experiments, respectively. The casting simulation results showed that the cooling performance of the CC was enhanced by ca. 10% compared with that of a conventional cooling channel. The experimental cooling performance of the CC improved by ca. 8% compared to that of a conventional cooling channel, and the increment in performance was consistent with the simulation results. In addition, microstructural evidence clarified that the effective cooling performance of CC could be attributed to the decrement (ca. 17%) of the secondary dendrite arm spacing (SDAS) of the Al-Si-Cu alloy. In this research, AM technology provides a novel way to fabricate functionally superior CC molds that are hardly producible with traditional methods. (Received May 14, 2021; Accepted June 4, 2021)

keyword : conformal cooling channel, additive manufacturing, casting simulation, Al-Si-Cu alloy casting, secondary dendrite arm spacing

In this study, we prepared highly thermostable semi-transparent heaters composed of W layers with thicknesses of 1-20 nm, on which a 30 nm-thick ZnO layer was deposited to serve as an anti-oxidation barrier. The optical transmittance and sheet resistance of the heaters could be greatly modulated by varying the W layer thickness. For layer thicknesses up to 10 nm, the initial Joule heating above 100 ℃ significantly reduced the sheet resistance, by 300% for a 6 nm-thick W layer at a fixed voltage for a duration of 400 s. During the test period, heater current and heating capability continuously increased. In subsequent heater operations, the heaters exhibited highly reproducible heating capability. In contrast, for films thicker than 10 nm, the Joule heating process resulted in only a marginal reduction in sheet resistance, i.e., by 4% for a 20 nm-thick W layer. In order to investigate the sharp dependence of heater characteristics on thickness, we performed x-ray diffraction analyses, which revealed that the films thinner than 10 nm were composed of both the equilibrium low-resistivity α-phase and metastable high-resistivity β-phase, and films thicker than 10 nm contained mostly α-phase. The Joule heating process for the thinner films was found to transform the β-phase into α-phase at temperatures above 100 ℃, which resulted in significant improvement in the heating capability of the 6 nm-thick W layer. For films thicker than 10 nm, the W layers contained mostly α-phase and no such transformation-induced effects were observed. Finally, W heaters composed of α-phase exhibited highly thermostable and reproducible heater properties, which make the heaters suitable for applications with semi-transparent heaters. (Received April 29, 2021; Accepted June 1, 2021)

keyword : tungsten, transparent heaters, thin films, sheet resistance, transmittance

In this work, MnO₂ nanoparticles were embedded in a carbon matrix as a porous composite, fabricated using a simple chemical route followed by low-temperature annealing, with activated carbon (AC) as the carbon source in the composite preparation. The porous MnO₂/carbon structures contained some selective nanoparticles coated with carbon. The structural feature was identified by transmission electron microscopy (TEM). The surface area and pore size distribution of the materials were investigated by N₂ adsorption/desorption isotherms, and demonstrated a high surface area of about 80 ㎡ g^-1. AC is a readily available carbon source that can easily form a composite with MnO₂ nanoparticles, forming a distinctive porous morphology. When employed as an anode material for lithium-ion batteries (LIB), the composite electrode demonstrated high specific capacities with an initial discharge capacity of 2500 mAh g^-1 and maintained about 1391 mAh g^-1 after fifty cycles. It also demonstrated excellent high rate performance, delivering more than 500 mAh g^-1 of specific capacity at 3000 mA g^-1, which is a higher capacity than a conventional graphite anode. Overall, the MnO₂/ carbon composite electrode delivered superior anode performance, which was attributed to the improved surface area of the carbon hybridized MnO₂ nanoparticles. The porous composite has benefits for lithium storage performance. (Received April 29 2021; Accepted June 21, 2021)

keyword : MnO_{2 nanoparticles, porous carbon composite, Li-ion battery, high specific capacities}

One-dimensional Al and Al₂O₃ nano/microstructures were fabricated via thermal oxidation of Al and SnO₂ powder mixtures at temperatures below the melting point of Al (660 ℃). Furthermore, the synthesis process was carried out in air at atmospheric pressure, which made the process very simple and easy. Sn metal particles with spherical shape were observed on the tips of the Al and Al₂O₃ nano/microstructures, suggesting that the nano/microstructures were grown via a catalyst-assisted growth mechanism. The Sn acted as a catalyst for growing the Al and Al₂O₃ nano/microstructures. The Sn with low melting point (232 ℃) was produced via the reduction of SnO₂ by Al, and formed catalyst droplets at the growth temperatures. Al atoms diffused and dissolved into the Sn liquid droplets, leading to the nucleation and then the growth of the Al and Al₂O₃ nano/microstructures. At 400 ℃, the diffusion of Al atoms into the Sn liquid droplets was associated with high stress generated at the Al₂O₃/Al interface. At 600 ℃ close to the melting point of Al, Al atoms were diffused from the thin Al liquid layer, which was formed on the surface of the Al powder, to the Sn liquid droplets. Simultaneously, the Al atoms reacted with oxygen in air and formed solid Al₂O₃ nuclei. A relatively strong ultraviolet emission band centered at 330 nm was observed in the sample prepared at 600 ℃. (Received May 5, 2021; Accepted June 2, 2021)

keyword : aluminum oxide, nano, microstructures, tin, catalyst, low temperature growth